fixes for the macOS build

- Adds a "fail" function parameter to execute_script (default is True).
- Disables failing for brew unlink execution.
- Unlinks gnutls, nettle, and msgpack which cause failed building
  when using homebrew cellar versions.
- Adds --enable-restbed when calling bootstrap on macOS.

Change-Id: I96449aaad57683f3bc778235376051a1f6ce69b1
Reviewed-by: Kateryna Kostiuk <kateryna.kostiuk@savoirfairelinux.com>